The theoretical basis for present day angular rate sensing was developed at least as early as 1850, when the French physicist, Jean Bernard Foucault, conducted a detailed investigation of the principles of conservation of momentum. In one experiment, he used a gyroscope to demonstrate the rotation of the earth. In another experiment, he discovered that the plane of transverse oscillation of a thin, vibrating rod clamped in the chuck of a lathe tends to remain fixed in space independent of the rotation of the chuck. Although these experiments clearly demonstrated the theoretical possibility for angular rate sensors, it was not until the early 20th century and the advent of instrument airplane flight that the necessity of developing a practical rate sensor was recognized.
Although from a theoretical standpoint the principles of conservation of momentum could be employed to produce an angular rate sensor using either a gyroscope or a vibrating element, gyroscopes proved to be much simpler to implement. Thus, the rate gyroscope naturally became the first commercially available type of angular rate sensor. However, even though it has been continually refined over the decades the rate gyro has several inherent limitations which can never be completely overcome. These include the sensing errors introduced by wear on the bearings which are essential to gyroscopic devices, and relatively high power consumption.
In response to the disadvantages of the rate gyro, many different types of vibrating element sensors have been developed, including vibrating wire, vibrating beam and vibrating rod sensors. Vibrating wire and beam sensors must be supported on both ends, and thus are particularly susceptible to sensing errors caused by thermal expansion and contraction, which have been found quite difficult to accommodate or compensate for. The vibrating rod sensor comprises at least one elongate vibrating element, of circular or rectangular cross section, which is fixed at one end to a mounting base and free on the other end. The rod is driven for vibration, typically at its resonant frequency, in a "drive" or "guide" plane such that under zero angular rotation conditions, the rod ideally has no component of motion normal or transverse to the drive plane. Typically, the element is constrained under zero angular rate conditions to movement in the drive plane by electromagnetic forces, mechancal configuration, material properties of the vibrating element, or a combination thereof. Thus, when the rod is rotated about its longitudinal or "sense" axis, the angular rate of turn may be detected by sensing the deflection of the rod out of the drive plane, or in other words the component of motion of the rod which is induced transverse to the drive plane, as caused by Coriolis forces. This transverse or "sense" motion is typically measured using piezoelectric bender elements which may be connected, or attached, or integral with the vibrating rod, or using electromagnetic elements such as magnetic forces and coils.
One important aspect of vibrating rod sensors concerns the vibrating characteristics of the rod. The natural resonant frequencies of the rod must be considered both for the purpose of controlling the frequency of vibration in the drive plane and for the purpose of providing predictable and measurable sense responses. The two resonant frequencies of concern are the drive plane resonant frequency and the sense motion resonant frequency. The peak of the sense response is at the resonant frequency of sense motions. Because energy from the drive plane which is converted to sense motion by angular rotations of the sensor is transferred at the drive resonance frequency, the best signal gain may be obtained by matching the drive and sense resonant frequency characteristics of the element. Thus, such a design has been adopted for many prior art devices. For instance, U.S. Pat. No. 2,513,340 to Lyman, U.S. Pat. No. 2,544,646 to Barnaby et al and U.S. Pat. No. 2,974,530 to Jaouen all teach that it is preferable that the drive and sense resonant frequencies be equal. There are, however, as will be demonstrated below in the ensuing specification, inherent difficulties and limitations in systems adopting this approach.
In addition to considerations relating to the resonant frequency characteristics, there are other aspects of vibrating rod sensor design which have a critical influence on the practical usefulness of such sensors. For instance, a sensor must be designed so that external shock vibration or the like do not cause undue extraneous and erroneous angular rate signals to be developed. Similarly, vibrating rod sensors must be designed such that vibrations from the drive plane are not transferred by acoustic or sonic energy propagation into sense motion, which also has the result of producing erroneous angular rate signals. Furthermore, in the case of wholly electromagnetic devices, i.e., devices that employ electromagnetics to both drive the element and to sense the rate of turn, cross coupling between the drive and sense coils must be accomodated or compensated for. Moreover, electromagnetic devices must also accommodate the effects of externally generated magnetic fields such as the earth's magnetic field.
There are other significant design barriers which must be overcome in order to provide a vibrating rod sensor that may be utilized over a wide range of temperatures, or in high shock environments. with regard to the former, materials and designs must have a low sensitivity to variation in temperature, both in terms of physical expansion and contraction of the components and in terms of the electromagnetic or electrical characteristics of the materials. Moreover, with regard to the latter, the materials must be selected to survive high G forces as are often found in military applications and in some commercial applications as well. Moreover, as is often necessary for military applications, a sensor may have to have a short warm-up time such that warm-up transients do not initially produce erroneous rate of turn measurements.
As will be seen from the ensuing specification, the present invention deals with each and every one of the above-mentioned design obstacles to provide a practical form of a vibrating rod sensor which is capable of performing up to exacting military standards or in less demanding commercial environments.